Catalytic converters have become important components of emission control systems for internal combustion engines. A catalytic converter is used to treat engine exhaust to reduce the amount of undesirable exhaust components released into the air. These undesirable exhaust components include carbon monoxide, hydrocarbons, and nitrogen oxides. A catalytic converter includes one or more catalysts that operate to speed up reactions converting the undesirable components into non-polluting components that are then released into the air, but generally requires relatively high temperatures to be effective. Using an oxidation catalyst, carbon monoxide is combined with oxygen and converted into carbon dioxide, and hydrocarbons are combined with oxygen and converted into carbon dioxide and water molecules. Using a reduction catalyst, nitrogen oxides are converted into nitrogen and oxygen. Generally, the amount of oxygen required for the oxidation reactions is more than that produced by the reduction reactions. Thus, an effective catalytic converter including both types of catalysts reduces the amount of oxygen at its output as compared to its input, as does an effective catalytic converter including only an oxidation catalyst.
Recent governmental regulations promulgated for the purpose of reducing air pollution have targeted motorcycles as well as automobiles. One way to monitor the efficiency of a catalytic converter in automobiles, for example, has been to include an oxygen sensor both upstream and downstream of the catalytic converter. A comparison is made of the output signals of the sensors to ensure that the amount of oxygen is reduced at the downstream sensor.